Residual Sea Level Research Articles

Some aspects of the seasonal sea level variations around Spain

Tide gauge stations are used to investigate the seasonal variability of sea level around Spain, which exhibits a well‐defined annual cycle and a small but distinguishable semiannual cycle in all stations. The contribution of different sources to observed sea level has been analyzed. The effect of wind and atmospheric pressure has been explored, analyzing sea level residuals computed with a barotropic ocean model forced by the meteorological fields. Sea level corrected for meteorological forcing contains only an annual signal, indicating that the semiannual cycle in the observed sea levels is mainly forced by these meteorological agents. Additional data sets (sea surface temperature satellite images and climatologies) are used to show that the annual cycle is related to the seasonal steric anomaly, which accounts for ∼60% of the cycle amplitude. The lack of agreement between the annual phases of sea level and steric anomaly cycles for the stations located in the continental margin of the Atlantic Ocean suggests the existence of other effects in the observed sea levels related to seasonal changes in ocean circulation. On the contrary, open‐ocean stations of the Canary Islands have sea level annual cycles phase locked with the steric anomaly cycles, although amplitudes differ significantly from one station to the other. This fact is interpreted as the local response to large‐scale seasonal fluctuations of the Canary Current. Sea level cycle in stations within the semienclosed Mediterranean Sea suggests the existence of a hydrological cycle that must include a seasonal barotropic signal through the Strait of Gibraltar.

Fundamental Adriatic seiche recorded by current meters

Abstract. Currents recorded at two stations positioned at the Adriatic shelf break between 17 February and 6 March 1989 were considered. They provided the first evidence of current variability related to the Adriatic-wide seiches. Current spectra were dominated by a peak at 21.1h – a well-known period of the fundamental Adriatic mode. Amplitudes of oscillations were considerable, occasionally greater than 30cm/s. Both along- and cross-basin currents were recorded, thus signalling the importance of rotational effects. Comparison of the current data with the sea-level measurements performed simultaneously along the east Adriatic coast showed that the 21-h currents flowing in the Adriatic preceded sea levels by a quarter of a cycle, as would be expected from a simple model of bay seiches. Sea-level amplitudes reached 40cm in the North Adriatic, thus marking the event as one of the strongest on record. Seiching was triggered by a suddenly changing sirocco wind, as is usually the case in the Adriatic. The most important features of the seiche event were reproduced with a two-dimensional hydrodynamic model of the Adriatic Sea, forced by the sea-level residuals measured at Otranto and the air pressure and wind data recorded at Lastovo. The model showed that the seiche-related currents were relatively strong in the area where the current measurements were performed and further north – between Zadar and Ancona. Key words. Oceanography: general (continental shelf processes) – Oceanography: physical (sea-level variations, currents)

Nonlinear Tidal Characteristics along the Uldolmok Waterway off the Southwestern Tip of the Korean Peninsula

Analyses of tidal observations and a numerical model of the <TEX>$M_2$</TEX> and <TEX>$M_4$</TEX> tides in the Uldolmok waterway located at the southwestern tip of the Korean Peninsula are described. This waterway is well known fer its strong tidal flows of up to more than 10 knots at the narrowest part of the channel. Harmonic analysis of the observed water level at five tidal stations reveals dramatic changes in the amplitude and phase of the shallow water constituents at the station near the narrowest part, while survey results show a decreasing trend in local mean sea levels toward the narrow section. It was also observed that the amplitudes of semi-diurnal constituents, <TEX>$M_2$</TEX> and <TEX>$S_2$</TEX> are diminishing toward the narrowest part of the waterway. Two-dimensional numerical modeling shows that the <TEX>$M_2$</TEX> energy flux is dominated by the component coming from the eastern boundary. The <TEX>$M_2$</TEX> energy is inward from both open boundaries and is transported toward the narrow region of the channel, where it is frictionally dissipated or transferred to other constituents due to a strong non-linear advection effect. It is also shown that the <TEX>$M_4$</TEX> generation is strong around the narrow region, and the abrupt decrease in the M4 amplitude in the region is due to a cancellation of the locally generated M4 with the component propagated from open boundaries. The superposition of both propagated and generated M4 contributions also explains the discontinuity of the M4 phase lag in the region. The tide-induced residual sea level change and the regeneration effect of the <TEX>$M_2$</TEX> tide through interaction with <TEX>$M_4$</TEX> are also examined.

Long-Term and Short-Term Variations of Sea Level in Eastern Canada: A Review

This is a review of sea level data performed at three selected stations (Québec-Lauzon, Harrington Harbour,and Halifax) in eastern Canada in order to investigate the seasonal trends and other long-term and short-term changes which occurred since the beginning of the 20th century. Stations situated in riverine or estuarine regions (e.g., Québec-Lauzon) are significantly affected by freshwater flow in their annual cycle of sea level changes and exhibit a definite maximum in spring and minimum in autumn-winter. Other stations situated in the eastern half of the Gulf of St. Lawrence (e.g., Harrington Harbour) or near the open Atlantic coast (Halifax) mainly follow the general cycle of subarctic regions, with lows in spring-summer and highs in autumn-winter. Such seasonal variations appear to be related to the atmospheric pressure and baroclinic current variations. Secular trends in mean sea level in eastern Canadian waterbodies show a mean rise of about 2.56 mm/yr -1 due to tectonic motions, that is, land subsidence. At several stations in eastern Canada, evidence is found for the influence of the nodal tide (18.6 years), the sunspot cycle (10.8 years), the lunar perigee (8.47 years), the pole tide (14.5 months), the annual cycle (12 months), and semiannual tidal cycle (6 months) in sea level records. Beside long-term oscillations with periods of more than one year, evidence is found for high energetic semidiurnal and diurnal tides where they contribute largely (from 90-95%) to short term variability of sea level. In the residual signal (variations of sea level--tidal variations), short-term variations between 2 to 30 days can be attributed to meterological forcing (atmospheric pressure and winds), longitudinal seiches (2-10 h), atmospheric tides (12 h and 24 h) and inertial oscillations (16-18 h). A regressive model showed that the water discharge from the St. Lawrence River contributes 29% to the monthly residual sea level at Québec-Lauzon. The atmospheric pressure and winds contribute respectively 8.1% and 8.9% at this station. They contribute 52.1% and 7.7% at Harrington Harbour and 41.8% and 14.3% at Halifax. The regression coefficients of residual sea level on atmospheric pressure are respectively estimated to be -1.507 cm. ( hPa ) -1 ( - 0.345 cm. ( hPa ) -1 ), -0.776 cm. ( hPa ) -1 ( - 0.112 cm. ( hPa ) -1 ) and -0.825 cm. ( hPa ) -1 ( - 0.008 cm. ( hPa ) -1 ) at the three stations. Compared to the coefficient of the inverted barometer, estimated to be -1 cm. ( hPa ) -1 , these effects of the atmospheric pressure on sea level variations seem to be amplified at Que´bec-Lauzon by the wind effects (and water discharge) while they are reduced at Harrington Harbour and Halifax.

Use of the wavelet transform on hydro-meteorological data

The advantages of utilising wavelet analysis to study the dependence of sea level variability on meteorological parameters are demonstrated on data collected at a sea level station on the northwestern coast of Malta. The strength of the method lies in the fact that time information is conserved and signal variability can be resolved up to levels which would equivalently require a much longer data set with Fourier transform methods. The validity of the wavelet transform is verified in its potential to decompose and identify in detail the synoptic variability of a parameter, as well as to follow the inter-dependence of more parameters by a quantitative comparison based on a correlation of their wavelet expressions. The results of discrete wavelet analysis exhibit significant differences in the temporal development of the wavelet decompositions of residual sea level. This is even more evident in the air pressure field for which the variability is principally explained by wavelet decompositions corresponding to central frequencies of 0.56 and 0.28 cpd. The reduced activity in barometric pressure fluctuations during summer is particularly evident at these two decompositions. This work also furnishes a contribution to studies concerning the validity of the inverted barometer effect. The position and extent of the frequency range at which the local sea level variation can be approximated by the inverted barometer effect is found to depend on the amplitude of the air pressure fluctuations. During the stronger pressure variations, the inverted correlation to the sea level occurs at practically the full range of synoptic frequencies and even lower.

Stochastic Modeling of Short Term Variations of Sea Level in Eastern Canada

Hourly sea level records from three stations in eastern Canada (Québec-Lauzon, Harrington-Harbour and Halifax) are analyzed both in frequency domain from 1970 to 1979 and in time domain during 1973. At the three stations, the deterministic model explains 90 to 96% of the total variability of sea level. The semidiurnal and diurnal tides contribute largely to its variations. The residual series, less than 10% of the initial variations of sea level, contain irregular values including extreme values of seiches and storm surges. Such random variations are analyzed and modeled following the method described by Box and Jenkins (1976). The long period variations (2 to 30 days) can be attributed to meteorological forcing (atmospheric pressure and winds). The short period variations (some hours to one day) can be attributed to longitudinal seiches, semidiurnal and diurnal atmospheric tides, and inertial oscillations. The water discharge from the St. Lawrence River contributes 29% of the monthly residual sea level at Qué

Complex empirical orthogonal functions analysis of ERS‐1 and TOPEX/POSEIDON combined altimetric data in the region of the Algerian current

Maps of sea level anomalies (SLA) relative to the 1993 annual mean sea level combine the data from the two altimetric missions, ERS‐1 and TOPEX/POSEIDON, during the overlap period (October 1992 to December 1993). These regular maps in space and time of residual sea level every 10 days on a 0.2° regular grid are used in the region of the Algerian current where the mesoscale eddies are of primary importance to the circulation of all the Mediterranean water masses. They are first compared with ERS‐1 along‐track scanning radiometer sea surface temperature images to get information on two anticyclonic eddies produced by instabilities of the Algerian current and visible in both infrared and altimetric data sets. Then, an analysis of complex empirical orthogonal functions (CEOFs) is performed on the SLA data set to see the correlation of the different dynamic features of the observed variability. The CEOF analysis is applied to the complex time series formed from the original SLA time series and their Hubert transforms to separate the variability into spatially coherent modes. The spatially correlated signal in the study area (0–15°E and 35°–40°N) was found to be dominated by the first two CEOFs. These first two modes explain nearly 85% of the variability, with 80% of the total variance for the first one and 5% of the total variance for the second one. The temporal phase of the first mode indicates that a constant frequency of one cycle per year is clearly dominant, corresponding to the seasonal signal. The strongest amplitude is obtained in the southern part of the channel of Sardinia and south of the Strait of Sicily. The temporal amplitude and the temporal phase of the second mode show a periodicity of about 6 months which appears to be associated with the variability of the Algerian current as the phase isolines are parallel to the mean current path along the Algerian coast. The strongest amplitude of the second mode is located near the African coast at ∼4°E and 8°E. These two points of high variability could correspond to eddy detachments from the main current.

Adriatic seiche decay and energy loss to the Mediterranean

A salient feature of sea level records from the Adriatic Sea is the frequent occurrence of energetic seiches of period about 21 h. Once excited by a sudden wind event, such seiches often persist for days. They lose energy either to friction within the Adriatic, or by radiation through Otranto Strait into the Mediterranean. The free decay time of the dominant (lowest mode) seiche was determined from envelopes of handpassed sea level residuals from three locations (Bakar, Split and Dubrovnik) along the Croatian coast during twelve seiche episodes between 1963 and 1986 by taking into consideration only time intervals when the envelopes decreased exponentially in time, when the modelled effects of along-basin winds were smaller than the error of estimation of decay time from the envelopes and when across-basin winds were small. The free decay time thus obtained was 3.2±0.5 d. This value is consonant with the observed width of the spectral peak. The decay caused by both bottom friction and radiation was included in a one dimensional variable cross section shallow water model of the Adriatic. Bottom friction is parameterized by the coefficient k appearing in the linearized bottom stress term ρ 0u (where u is the along-basin velocity and ρ 0 the fluid density). The coefficient k is constrained by values obtained from linearization of the quadratic bottom stress law using estimates of near bottom currents associated with the seiche, with wind driven currents, with tides and with wind waves. Radiation is parameterized by the coefficient f appearing in the open strait boundary condition ζ = auh/ c (where ζ is sea level, h is depth and c is phase speed). This parameterization of radiation provides results comparable to allowing the Adriatic to radiate into an unbounded half plane ocean. Repeated runs of the model delineate the dependence of model free seiche decay time on k and a, and these plus the estimates of k allow estimation of a. The principle conclusions of this work are as follows. 1. (1) Exponential decay of seiche amplitude with time does not necessarily guarantee that the observed decay is free of wind influence. 2. (2) Winds blowing across the Adriatic may be of comparable importance to winds blowing along the Adriatic in influencing apparent decay of seiches; across-basin winds are probably coupled to the longitudinal seiche on account of the strong along-basin variability of across-basin winds forced by Croatian coastal orography. 3. (3) The free decay time of the 21.2 h Adriatic seiche is 3.2±0.5 d. 4. (4) A one dimensional shallow water model of the seiche damped by bottom stress represented by Godin's (1988) approximation to the quadratic bottom friction law ρ 0 C D u| u| using the commonly accepted drag coefficient C D = 0.0015 and quantitative estimates of bottom currents associated with wind driven currents, tides and wind waves, as well as with the seiche itself with no radiation gives a damping time of 9.46 d; radiation sufficient to give the observed damping time must then account for 66% of the energy loss per period. But independent estimates of bottom friction for Adriatic wind driven currents and inertial oscillations, as well as comparisons between quadratic law bottom stress and directly measured bottom stress, all suggest that the quadratic law with C D=0.0015 substantially underestimates the bottom stress. Based on these studies, a more appropriate value of the drag coefficient is at least C D=0. In this case, bottom friction with no radiation leads to a damping time of 4.73 d, radiation sufficient to give the observed damping time then accounts for 32% of the energy loss per period.

The Frequency Distributions of the ReSidual Sea Level along the Suez Canal

The Frequency Distributions of the ReSidual Sea Level along the Suez Canal

Present‐day post‐glacial sea level change far from the Late Pleistocene ice sheets: Implications for recent analyses of tide gauge records

We examine the sensitivity of predictions of the present‐day rate of sea level change due to glacial isostatic adjustment (GIA) in two regions located within the ‘far‐field’ of the Late Pleistocene ice sheets (Europe/north Africa and the western Pacific/Caribbean) to variations in the mantle viscosity profile. The regions considered here encompass the location of 11 (of 21) sites used in a recent determination, based on GIA‐corrected tide gauge records, of the global rate of secular sea level change. Our analysis suggests that the mean GIA correction at the 11 far‐field sites may vary by as much as 0.5 mm/yr, depending on the adopted Earth model, and we derive a bound on the (residual) secular sea level rise of 1.1 to 1.6 mm/yr. This bound is consistent with our analysis of the tide gauge record from the U.S. east coast, and it suggests that previous estimates of the residual sea level rise represent an upper bound.

Platform Harvest sea level measurement comparisons

Three in situ water level measurement systems have been installed on Platform Harvest in support of the TOPEX/Poseidon Satellite Verification Experiment. Each of the systems have fundamentally different instrument configurations, which provides a unique simultaneous set of measurements in a deep water‐exposed near‐shore environment. Each of the sensor “zeros”; have also been referenced to the same datum as a requirement of the experiment; thus absolute differences can be analyzed. Straightforward, simple comparisons of the systems are presented for TOPEX/Poseidon cycles 2 through 30. The systems are shown to agree well both in the mean and in frequency content. Nontidal sea level residuals are shown to be highly correlated to atmospheric pressure.

Correlated environmental corrections in TOPEX/POSEIDON, with a note on ionospheric accuracy

Estimates of the effectiveness of an altimetric correction, and interpretation of sea level variability as a response to atmospheric forcing, both depend upon assuming that residual errors in altimetric corrections are uncorrelated among themselves and with residual sea level, or knowing the correlations. Not surprisingly, many corrections are highly correlated since they involve atmospheric properties and the ocean surface's response to them. The full corrections (including their geographically varying time mean values), show correlations between electromagnetic bias (mostly the height of wind waves) and either atmospheric pressure or water vapor of −40%, and between atmospheric pressure and water vapor of 28%. In the more commonly used collinear differences (after removal of the geographically varying time mean), atmospheric pressure and wave height show a −30% correlation, atmospheric pressure and water vapor a −10% correlation, both pressure and water vapor a 7% correlation with residual sea level, and a bit surprisingly, ionospheric electron content and wave height a 15% correlation. Only the ocean tide is totally uncorrelated with other corrections or residual sea level. The effectiveness of three ionospheric corrections (TOPEX dual‐frequency, a smoothed version of the TOPEX dual‐frequency, and Doppler orbitography and radiopositioning integrated by satellite (DORIS) is also evaluated in terms of their reduction in variance of residual sea level. Smooth (90–200 km along‐track) versions of the dual‐frequency altimeter ionosphere perform best both globally and within 20° in latitude from the equator. The noise variance in the 1/s TOPEX ionospheric samples is ∼(11 mm)2, about the same as noise in the DORIS‐based correction; however, the latter has its error over scales of order 103 km. Within 20° of the equator, the DORIS‐based correction adds (14 mm)2 to the residual sea level variance.

Observations on the long‐period variability of the Gulf Stream downstream of Cape Hatteras

To examine the long‐period variability of the Gulf Stream, sea level residuals relative to a 2‐year mean sea level in the Gulf Stream downstream of Cape Hatteras (between 75°W and 60°W longitude) are used. Residuals, as derived from Geosat altimetry between November 1986 and December 1988, were gridded in space and time at a temporal resolution of 10 days and spatial resolution of 1/4°. Complex empirical orthogonal function (CEOF) analysis was applied to the data set to extract the spatially correlated signal with the original data sub sampled to 1/2°. In addition to determining the space‐time scales and propagation characteristics of the different modes, wavenumber‐frequency spectral techniques were used to separate the variability into propagating and stationary components. The CEOF technique applied to the data set indicated that the first four CEOF modes accounted for 60% of the variability and were found to be above the noise level 99% of the time. CEOF 1 was associated with westward propagation at 5 km/d at a wavelength of 2000 km and eastward propagation at 1–2 km/d centered at a 500‐km wavelength. This first CEOF is in good agreement with thin‐jet equivalent barotropic models which predict westward propagation for wavelengths greater than 1130 km. A deflection of the wavelike pattern at 65°W also indicates a possible topographic effect. A simple scaling of the effect of topography indicates that for length scales longer than the internal Rossby radius of deformation, the topographic term is at least of the same order of magnitude as the beta effect. The second CEOF was more broadbanded in wavenumber space, with eastward propagation occurring in a wavenumber‐frequency band between 300 and 1400 km and 0.5 and 2.0 cycles/yr. The third CEOF is similar in structure to the first, but with less energy. CEOF 4 was clearly identifiable with higher frequencies than the first three with westward propagation at 4 km/d. The spatial location of this mode along with the westward propagation indicates possible influences from eddy‐stream interactions. Thus topography, Rossby wave dynamics, and eddy‐stream interactions all appear to have a significant role in determining the space‐time scales and propagation properties of the long‐period response of sea level in the Gulf Stream.

Seasonal fluctuations in sea level on the South Carolina shelf and their relationship to the Gulf Stream

Near‐bottom pressure measurements obtained over a 9‐month period in 1978 on the outer continental shelf off South Carolina document that sea level rose 35 to 50 cm between July and October. Records of coastal sea level showed a similar rise. When the pressure and coastal sea level records were corrected for the effects of wind stress and temperature, the largest portion of the rise in the two variables remained. The amplitude of the change in residual sea level was largest at the shelf break, 28 cm, and decayed to 22 cm at the coast. The relatively small magnitude of the decay suggests that alongshelf pressure gradients imposed at the shelf break have minimum alongshelf wavelengths of 2000 km. The changes in residual sea level were seasonal and associated with seasonal changes in the transport of the Gulf Stream. The 3 years of residual sea level records examined here indicate that the amplitude of a typical decrease in transport of the Gulf Stream off South Carolina between July and October has a strong interannual variability. The change in transport can be 50% lower to over 100% higher than the historical average. The residual sea level records also suggest that a change in transport of the Gulf Stream is not strongly related to the onshore/offshore position of the Gulf Stream.

The mean sea surface height and geoid along the Geosat subtrack from Bermuda to Cape Cod

Measurements of near‐surface velocity and concurrent sea level along an ascending Geosat subtrack were used to estimate the mean sea surface height and the Earth's gravitational geoid. Velocity measurements were made on three traverses of a Geosat subtrack within 10 days, using an acoustic Doppler current profiler (ADCP). A small bias in the ADCP velocity was removed by considering a mass balance for two pairs of triangles for which expendable bathythermograph measurements were also made. Because of the large curvature of the Gulf Stream, the gradient wind balance was used to estimate the cross‐track component of geostrophic velocity from the ADCP vectors; this component was then integrated to obtain the sea surface height profile. The mean sea surface height was estimated as the difference between the instantaneous sea surface height from ADCP and the Geosat residual sea level, with mesoscale errors reduced by low‐pass filtering. The error estimates were divided into a bias, tilt, and mesoscale residual; the bias was ignored because profiles were only determined within a constant of integration. The calculated mean sea surface height estimate agreed with an independent estimate of the mean sea surface height from Geosat, obtained by modeling the Gulf Stream as a Gaussian jet, within the expected errors in the estimates: the tilt error was 0.10 m, and the mesoscale error was 0.044 m. To minimize mesoscale errors in the estimate, the alongtrack geoid estimate was computed as the difference between the mean sea level from the Geosat Exact Repeat Mission and an estimate of the mean sea surface height, rather than as the difference between instantaneous profiles of sea level and sea surface height. In the critical region near the Gulf Stream the estimated error reduction using this method was about 0.07 m. Differences between the geoid estimate and a gravimetric geoid were not within the expected errors: the rms mesoscale difference was 0.24 m rms.

Sea level variabilities in the Gulf Stream between Cape Hatteras and 50°W: A Geosat study

Sea level residuals relative to a 2‐year mean sea level in the Gulf Stream downstream of Cape Hatteras (between 80°W and 50°W longitude) are studied using Geosat altimetry between November 1986 and December 1988. Spectral slopes computed at four nominal locations along the Gulf Stream (72°W, 65°W, 60°W, and 55°W) range between −1.5 and −0.9, significantly less steep than the −2.5 spectral slope found at Bermuda. Because of increased short‐wavelength energy, the spectra tend to become more “white” going eastward, but this trend is not statistically significant. Energy at the annual cycle decreases eastward from around 40% of the total variance at 72°W to 20% of the total variance at 55°W. Maximum residual geostrophic sea surface velocities associated with the annual cycle were of the order of 70 cm/s. Comparison between the Gulf Stream path from sea level residual maps and the north wall edge observed from NOAA infrared (IR) imagery shows an rms difference of 49 km between 75°W and 60°W longitude, but not all sea level maps show a clearly defined Gulf Stream. Using the IR estimates for Gulf Stream position, sea level residuals were modeled as the meandering of a sea level front shaped as 0.5 m · tanh {(X–X0)/[50 km · sin (α)]}, with α the angle between the Geosat track and the Gulf Stream edge and X0 is based on the observed IR north wall positions. This showed that the annual signal, in terms of the location and phase of its maxima, is dominated by the meandering of the Gulf Stream.

Annual Rossby Wave Characteristics in the California Current Region from the GEOSAT Exact Repeat Mission

Abstract Examination of sea level variability in the California Current region, from 20° to 50°N and from the coast of North America to 140°W, was made possible with altimetric observations obtained by the GEOSAT Exact Repeat Mission (ERM) for the period November 1986-October 1987. During this period, a time sequence of twenty-one 17-day maps of altimetric sea level residuals over the Califomia Current region was constructed. The rms distribution of these altimetric sea level residuals displayed local maxima at three locations adjacent to the coast of California; i.e., off Point Eugenia at 27°N, southwest of Point Conception at 32°N, and between Monterey and Cape Mendicino (i.e., 37°–40°N). Complex empirical orthogonal function (CEOF) analysis reveals that 53% of the variance over this one-year period was associated with the annual cycle, with amplitude maximum in summer and minimum in winter. The spatial distribution of amplitude displayed local maxima adjacent to the coast, again at 27°N, 32°N, and from...

Continuous assimilation of Geosat altimetric sea level observations into a numerical synoptic ocean model of the California Current

In this study, real Geosat altimetric sea level observations for the 1‐year period extending from January to December 1987 were assimilated into a realistic wind‐driven numerical synoptic ocean model of the California Current. The objective was to evaluate the effectiveness of using a realistic synoptic ocean model to interpolate (dynamically) real altimetric sea level observations from the Geosat ERM onto a regular grid. First, the model mean sea level was shown to simulate qualitatively the observed mean sea level found in in situ CaICOFI hydrographic data. Second, the model was shown to simulate approximately the space‐time statistics of mesoscale eddy variability contained within the altimetric sea level observations. Third, prior to assimilation the altimetric sea level observations were referenced using this model mean. Then the referenced altimetric sea level observations were assimilated into the model, with the resulting sea level residuals compared with estimates from in situ (expendable bathythermograph) observations. This comparison yielded nearly exact agreement at low frequency (i.e., the semiannual cycle), but less agreement on month‐to‐month time scales of variability (yet still significant), probably owing to the unaltered nature of the in situ estimates. Subsequent comparisons of the dynamically interpolated altimetric sea level residuals with those obtained from the statistical interpolation yielded marked improvement in the latter over the former; i.e., dynamical interpolation allowed observed mesoscale variability to be conducted from the Geosat ERM repeat tracks into the regions between tracks with little change in magnitude and structure, not possible with the statistical interpolation. The latter tended to produce extrema on grid points that coincided with the track lines. As such, dynamical interpolation demonstrated marked improvement in resolution, time‐space continuity, intensity, and gradient structure of mesoscale eddy activity over that possible with statistical interpolation. Model‐data assimilation also allowed for the determination of the vertical structure of the mesoscale eddy activity. Moreover, it yielded a forecasting capability that had a statistically significant improvement over persistence in specifying mesoscale eddy activity 2–3 weeks into the future.

Currents through Torres Strait

Abstract A five-month field study of the circulation in the Torres Strait was carried out. Baroclinic effects were negligible. The Arafura Sea and the Coral Sea forced a different tide on either side of Torres Strait, resulting in fluctuations of sea level difference of up to 6 m on either side of the Strait. The tidal dynamics in the Strait were controlled by a local balance between the acceleration, the sea level slope, and the bottom friction. Only 30% of the semidiurnal tidal wave was transmitted through Torres Strait. There were also fluctuations of the high-frequency sea level residuals (up to 0.8 m peak to trough) which appeared to be related to complex flows both through the Strait and across the Strait. Low-frequency sea level fluctuations were incoherent on either side of the Strait, and resulted in fluctuations of the low-frequency sea level differences on either side of the Strait of typically 0.3 m. These sea level gradients and the local wind forcing generated low-frequency current fluctuati...

Fluctuations of monthly sea level as related to the intensity of the Gulf Stream from Key West to Norfolk

Tide gauge data from Key West to Norfolk were used to identify a monthly signal in sea level that is uncorrelated with local shelf‐trapped processes. Time series (1955–1975) of local winds, sea level slope, and river runoff were used in a regression model of sea level to separate a local response and a residual signal. The monthly means of the residual contribution were investigated for their relationship to the seasonal fluctuations of the Gulf Stream. In the Florida Channel, lower sea level is found to correspond to increased flow of the Florida Current. During July and August a marked fall in residual sea level, unrelated to the coastal winds, is found from Key West to Charleston, suggesting that the transport of the Gulf Stream increases both in and north of the Florida Channel during this time. Measured long‐term monthly surface currents at Diamond Shoals, Cape Hatteras, which markedly increase in summer to high velocities, tend to substantiate this claim. An additional wintertime low in residual sea level occurs north of the Florida Channel. The wintertime low does not result from steric heating within the upper 100–150 m of water, nor does it, in contrast to the summertime low, appear to coincide with increased northward surface flow. Monthly mean Sverdrup transport was computed across the North Atlantic between 15°N and 35°N. When compared at the same latitude, residual sea level and Sverdrup transport, both interpreted as indices of Gulf Stream transport, generally disagree in phase during summer. However, north of the Florida Channel they are consistent during winter, assuming that lower sea level at this time reflects increased flow in the stream.